Periodic Density Functional Theory Study of the Structure, Raman Spectrum, and Mechanical Properties of Schoepite Mineral

Abstract

The structure and Raman spectrum of schoepite mineral, [(UO)O(OH)]·12HO, was studied by means of theoretical calculations. The computations were carried out by using density functional theory with plane waves and pseudopotentials. A norm-conserving pseudopotential specific for the U atom developed in a previous work was employed. Because it was not possible to locate H atoms directly from X-ray diffraction (XRD) data by structure refinement in previous experimental studies, all of the positions of the H atoms in the full unit cell were determined theoretically. The structural results, including the lattice parameters, bond lengths, bond angles, and powder XRD pattern, were found to be in good agreement with their experimental counterparts. However, the calculations performed using the unit cell designed by Ostanin and Zeller in 2007, involving half of the atoms of the full unit cell, led to significant errors in the computed powder XRD pattern. Furthermore, Ostanin and Zeller's unit cell contains hydronium ions, HO, which are incompatible with the experimental information. Therefore, while the use of this schoepite model may be a very useful approximation requiring a much smaller amount of computational effort, the full unit cell should be used to study this mineral accurately. The Raman spectrum was also computed by means of density functional perturbation theory and compared with the experimental spectrum. The results were also in agreement with the experimental data. A normal-mode analysis of the theoretical spectra was performed to assign the main bands of the Raman spectrum. This assignment significantly improved the current empirical assignment of the bands of the Raman spectrum of schoepite mineral. In addition, the equation of state and elastic properties of this mineral were determined. The crystal structure of schoepite was found to be stable mechanically and dynamically. Schoepite can be described as a brittle material exhibiting small anisotropy and large compressibility in the direction perpendicular to the layers, which characterize its structure. The calculated bulk modulus, B, was ∼35 GPa.

Becquerelite mineral phase: crystal structure and thermodynamic and mechanical stability by using periodic DFT

“…[47][48][49][50] The hydrogen atom positions in the structure of schoepite were successfully determined using theoretical methods in a previous work. 88 The unit cell of becquerelite mineral phase, including the hydrogen atom positions, was fully optimized in this work. The calculations were performed using theoretical solid-state methods based on density functional theory using plane waves and pseudopotentials.…”

“…Once the thermodynamic properties of these materials were known, they were used in order to derive the enthalpy and Gibbs free energy of formation of becquerelite in terms of the elements using the methods developed in recent works. [90][91][92] These thermodynamic properties of formation were then combined with those of other important uranyl-containing materials, dehydrated schoepite (UO 2 (OH) 2 ), soddyite ((UO 2 ) 2 -(SiO 4 )$2H 2 O), rutherfordine (UO 2 CO 3 ) and gamma uranium trioxide (g-UO 3 ), 90 to study the four reactions:…”

Silver Oxalate: Mechanical Properties and Extreme Negative Mechanical Phenomena

“…[77] The calculated elasticity matrix, C i j , is reported in Table S5, Supporting Information. As in previous studies, [1,2,5,6,[83][84][85][86] the Reuss approximation was chosen as the best approach in the case of silver oxalate because it provided the best comparison of the results for the calculated bulk modulus, B = 9.64 ± 2.51 GPa, with that determined from the EOS. For monoclinic crystals, the generic necessary and sufficient Born criterion is that all eigenvalues of the C matrix be positive.…”

“…The mechanical properties of polycrystalline silver oxalate were determined using the Voigt, [80] Reuss, [81] and Hill [82] schemes. As in previous studies, [1,2,5,6,[83][84][85][86] the Reuss approximation was chosen as the best approach in the case of silver oxalate because it provided the best comparison of the results for the calculated bulk modulus, B = 9.64 ± 2.51 GPa, with that determined from the EOS. The results are reported in Table 1.…”

Revealing hydrogen atoms in a highly-absorbing material: an X-ray diffraction study and Torque method calculations for lead-uranyl-oxide mineral curite

“…5,13 There are just four minerals known to contain H 2 O molecules in the interlayer only; their structures are stabilized by an extensive network of H-bonds (schoepite, metaschoepite, paulscherrerite and heisenbergite). [14][15][16][17][18] In general, the thermodynamic stability of the structures of layered hydrated oxysalts is driven by the weak bonding interactions taking place between the rigid parts of the structure (called structure units) and interlayer regions usually containing only hydrated cations or H 2 O molecules. [19][20][21][22] Resolving hydrogen locations in crystal structures of hydrated uranium oxysalts is thus crucial for advancing our understanding of the thermodynamic stability and thus potential release of uranium into the environment.…”